The present invention relates to a circuit used in time spectroscopy, which is the measurement of the time relationship between the occurrence of two nuclear events. The present invention is particularly related to a circuit which utilizes passive circuit components only.
In the measurement of the time relationship between the occurrence of two nuclear events, it is known to be extremely difficult to obtain a precise and accurate signal indicative of the occurrence of each event. For instance, the pulses derived from a radiation detector may be different in wave shape and amplitude depending upon the type of radiation particle detected, the characteristics of the detector and other factors unique to the measurement process. The signal resulting from the appropriate amplification of the pulse from the detector is used to trigger a circuit for generating a logic signal to establish the time of occurrence of each event. Ideally, the timing of the logic pulse should be insensitive to the shape and to the amplitude of the trigger pulse.
One method used in the prior art is known as the constant-fraction method. Because the timing of events is important in scintillator/photomultiplier systems, a time-pickoff circuit was designed that would trigger an output signal at the same fraction of the input pulse amplitude, regardless of the total pulse height. The fraction of the pulse height was selected as the one at which the best time resolution could be obtained.
The constant-fraction method is applied to an input signal by delaying the signal and substracting a fraction of the undelayed pulse signal from it. A bipolar pulse is thereby generated, and its zero-crossing is detected and used to produce the output logic pulse. It has been possible to make the zero-crossing time of such a bipolar timing signal insensitive to amplitude and rise time variations of the input signals to the circuit. Consequently, the constant-fraction method is one of the most widely used techniques available in the timing spectroscopy art.
There have been several signal forming techniques used in the prior art typified by the circuits shown respectively in FIGS. 1 and 2. In the circuit of FIG. 1 the constant-fraction signal is formed at a low impedance, forward biased diode junction. This circuit is limited by the bandwidth and dynamic range considerations of the active electronic summing circuitry. A similar method and circuit is shown and described in detail in U.S. Pat. No. 3,818,356.
The circuit of FIG. 2 uses a differential amplifier at the output to perform both the inverting and summing functions. The output signal formed by such a circuit is limited by noise, bandwidth considerations and dynamic range capability of the active elements. A somewhat similar circuit to the device of FIG. 2 is shown and described in U.S. Pat. No. 3,763,436.
The accuracy of a constant-fraction timing circuit is directly related to the quality of the bipolar timing pulse which is formed. The particular circuit to be used to form the constant-fraction bipolar signal should have the following characteristics: (1) the circuit should be relatively free of noise so that the zero-crossing time can be accurately detected; (2) the circuit should not be limited by high-frequency bandwidth considerations so that time jitter and amplitude-dependent time walk of the zero-crossing point can be reduced or avoided; and (3) to reduce amplitude dependent time walk, the circuit should not be limited by dynamic range.